FIG. 1 is a block diagram for schematically showing the internal construction of an electric power steering (EPS) system for vehicles. The electric power steering system includes a torque sensor 110, an electronic controller unit (ECU) 120 and an EPS motor 130. The ECU 120 then comprises a micro controller unit (MCU) 122, a motor driver 124, a conventional interlock apparatus 126 and a current sensor 128. Of these components, the conventional interlock apparatus 126 will inhibit the operation of the electric power steering system in case of a system abnormality for the sake of the drivers' safety.
To be more specific, the conventional interlock apparatus 126 monitors the control of the EPS motor 130 by the MCI 122 whether it is carried out within normal premises (control-allowed area) as illustrated in FIG. 2 where a comparison is made between the torque sensor output and the feedback current value back from the EPS motor 130 to see when the EPS motor 130 output with respect to the torques sensor output resides in abnormal premises (control-inhibit area) to override the control by the MCU 122 and normalize the EPS motor 130 output torque toward the normal premises.
If the EPS motor 130 were of 3-phase, even with its output torque remained constant the feedback current value detected by current sensor 128 will characteristically vary as the motor's rotor changes position.
The phase current value of the 3-phase motor comprises phase currents ia, ib and ic as shown in FIG. 3 where the conventional interlock apparatus 126 has been arranged to represent the output torque of the EPS motor 130 with one of these phase current values through Clark transformation and Park transformation performed on 3-phase feedback current values using a precise rotor position “θ” of FIG. 3 of the motor to convert the 3-phase feedback current or the three parameters ia, ib and ic into two types of current value or a value of two phases which are an output torque current value (see FIG. 3 iq) and a reactive current value (id), whereby with the reactive current remained constant the output torque current value was made to represent the output torque of the EPS motor 130. Here, the output torque current value means the amount of the effective current of the supplied current to generate the output torque of the EPS motor 130 and the reactive current means the current portion supplied to EPS motor 130 contributing none to the output torque.
This necessitated the conventional interlock apparatus 126 of the electric power steering system to perform calculations shown in FIG. 3 and process signals of the highly precise motor position sensor such as a relative angular sensor. In order to realize the FIG. 3 calculation and the signal processing of such precision motor positional sensor, a high performance processor was needed.
Therefore, the high performance processor used with the conventional interlock apparatus 126 bounds to lift the unit manufacturing cost of the electric power steering systems.